The invention relates to optical storage systems, and more particularly, to an integrated laser driver signal processor circuit installed in an optical pickup unit of an optical storage system.
In the field of products related to optical discs, such as compact discs (CDs), digital versatile/video discs (DVDs) and the like, an increase in capacity of the optical disc and an increase in speed of data transfer of the optical disc storage system are continuously being desired. Also, as the capacity of optical discs is increased, mark and space (corresponding to information of 1 and 0) to be formed on the optical disc by the optical disc apparatus are required to be finer and more accurate.
In order to form accurate and fine mark and space, a mark recording waveform is required to have a multi-pulse form to be used as a drive current waveform for a semiconductor laser during recording. A pulse position or pulse width at the mark start and a pulse position or pulse width at the mark termination must be adaptively controlled in accordance with a mark length and an adjoining space length. For example, according to the DVD-RAM specification, the aforementioned adaptive control of the pulse position or pulse width is required to be performed in T/16 to T/32 steps (T being a minimum unit for determining the mark and space lengths and corresponding to the period of the so-called channel clock chCLK).
Furthermore, in contrast to the conventional binary format, the drive current waveform is also needed to be in a quaternary form and is therefore more complicated. As the data transfer speed is increased, the frequency of the aforementioned drive current waveform becomes higher.
Because the capacity and data transfer speeds are increased in this manner, current at multiple-valued levels supplied to the semiconductor laser must be switched at a high speed. To ensure such a high-speed current switching characteristic (rise characteristic: Tr, and fall characteristic: Tf of the drive current), it is preferable that the semiconductor laser drive circuit be disposed in the proximity of the semiconductor laser.
To meet this requirement, a conventional semiconductor laser drive circuit is so constructed as to have a plurality of current sources at least one of which is externally selected to drive the semiconductor laser. Therefore, as the drive current waveform has multiple valued levels, the number of control signal lines for selecting the current source increases. When the semiconductor laser drive circuit is carried on an optical pickup, signals are supplied to the optical pickup and received from the optical pickup through a flexible cable. However, as the capacity and speed of the optical disc increases, the level of the reproduction signal for reading information from the recording medium decreases and a difficulty arises that because of degradation of the control signal waveforms and the difference (skew) in delay amounts between control signals, the accurate drive current waveform cannot be obtained. For the sake of assuring reliable reproduction of data, sources of signal degradation such as cross-talk from the recording circuit system to the reproduction circuit system must be further decreased. The optical pickup carries photo detectors for detecting a reflection light beam from the optical disc and I-V amplifiers for converting output currents of the photo detectors to voltages, and outputs of the amplifiers are supplied to a decoder through the flexible cable.
For example, FIG. 1 shows a block diagram of an optical drive 100 according to the related art. The optical drive 100 includes a control unit 102 and an optical pickup unit 104 connected by a flexible cable 106. As shown in FIG. 1, the control unit 102 includes a controller such as a digital signal processor (DSP) 108, an encoder unit 110, a record phase lock loop (PLL) 112, a write strategy timing generator 114, an automatic power control (APC) circuit 116, a decoder unit 118, a receive PLL 120, and an RF signal processor 122 including a wobble processor 124. The optical pickup unit 104 includes a first laser diode LD1, a second laser diode LD2, a front monitor diode (FMD), a photo detector integrated circuit PDIC 126, a current to voltage converter 128, and a laser current driver generator 130. The optical pickup unit moves across an optical medium 132 and is connected to the control unit 102 via a flexible cable 106. To allow the optical pickup to move while maintaining electrical connectivity, all control signals of the laser current driver generator 130 and all electrical signals corresponding to the light received by the laser diodes LD1, LD2 and the front monitor diode FMD are coupled to the control unit 102 using the flexible cable 106. Accordingly, at high speeds, the influence of cross-talk and other distortion sources on the flexible cable 106 on the signals transmitted to the control unit 102 or to the optical pickup unit 104 must be reduced to prevent degradation of the reproduction signal to noise radio S/N.